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The Solar System • Section Summary The Sun

Key Concepts • What are the three layers of the sun's interior? • What are the three layers of the sun's atmosphere? • What features form on or above the sun's surface?

The sun's mass is 99.8 percent of all the mass in the solar system. Because the sun is so large, its gravity is strong enough to hold all of the planets and other distant objects in orbit. Unlike Earth, the sun does not have a solid surface. Like Earth, the sun has an interior and an atmosphere. The sun's interior consists of the core, radiation zone, and convection zone. Each layer has different properties. The sun produces an enormous amount of energy in its core, or central region. The sun's energy comes from nuclear fusion. In the process of nuclear fusion, hydrogen atoms in the sun join to form helium. The light and heat produced by the sun's core first pass through the middle layer of the sun's interior, the radiation zone. The radiation zone is a region of very tightly packed gas where energy is transferred mainly in the form of electromagnetic radiation. The convection zone is the outermost layer of the sun's interior. Hot gases rise from the bottom of the convection zone and gradually cool as they approach the top. Cooler gases sink, forming loops of gas that move heat toward the sun's surface. The sun's atmosphere consists of the photosphere, the chromosphere, and the corona. The inner layer of the sun's atmosphere is called the photosphere. Photo means "light," so the photosphere is the sphere that gives off visible light. At the beginning and end of a solar eclipse, you can see a reddish glow around the photosphere. This glow comes from the middle layer of the sun's atmosphere, the chromosphere. Chromo means" color," so the chromosphere is the "color sphere." During a total solar eclipse, a fainter layer called the corona is visible. The corona sends out a stream of electrically charged particles called solar wind. Features on or above the sun's surface include sunspots, prominences, and solar flares. Sunspots are areas of gas on the sun that are cooler than the gas around them. Sunspots usually occur in groups. Reddish loops of gas called prominences link different parts of sunspot regions. Sometimes the loops in sunspot regions suddenly connect, releasing large amounts of energy. The energy heats gas on the sun to millions of degrees Celsius, causing the gas to explode into space. These explosions are known as solar flares. Solar flares can greatly increase the solar wind.

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